Field of the Invention
This invention relates to a calender stack, including at least two rollers, the at least two rollers mounted in a housing, between which a nip can be formed, and the ends of the rollers are supported in rotary fashion in bearings and one of the rollers is stationary and at least one other roller is an advancing roller and has a first adjusting system by which the bearings of the advancing roller can be moved, thus changing the nip.
Discussion of Related Art
Calender stacks of this kind are used, for example, in the manufacture of plastic sheets and plates. A molten mass produced in an extrusion process, after emerging from a sheet die, is guided through an adjustable calender nip between the rollers. The molten mass is cooled in the process and shaped so that a sheet web is produced with a uniform thickness and a homogeneous appearance across the entire sheet surface. So-called calendered sheets of this kind are made, for example, of polystyrene, polypropylene, or polyester and are used among other things for transporting and protecting foods and are also used in the automotive industry.
A calender stack normally includes at least two rollers, one of which is stationary and at least one other roller is embodied as an advancing roller. This advancing roller can be moved by an adjusting system so that different calender nip settings can be produced between the stationary roller and the adjustable one. In this way, it is possible to produce sheets of different thicknesses.
Because of numerous interfering influences, for example inevitable roller deflections that also become more evident with increasing dimensions of the rollers, it can be difficult to ensure a uniform desired thickness of the film produced in the calender stack across its entire width. Consequently, attempts have already been made to measure the thickness of the film across its width and to produce a thickness profile. Subsequently, based on the determined thickness profile, influence can be exerted on the thickness profile of the molten mass supplied to the calender stack in order to counteract the undesirable effects that are caused by the roller deflection. It is thus possible to change the outlet opening of a sheet die in accordance with the thickness measurement profile in order to match the thickness profile of the resulting film as constantly as possible to a predetermined setpoint value. This entails a high degree of control complexity, which is considered disadvantageous.